Typically it's money set aside for a person as inheritance or money for getting started in life, placed 'In Trust', so that it cannot be accessed until certain conditions are met, like turning 18/21, graduation of some kind, getting married, etc. People do this to pass money along without dumping it all on someone before they're ready for the responsibility. I've never met anybody who had one that I knew about, but some people don't share that stuff as eagerly - it might make you look like a cream puff if others think you can't/don't have to make it on your own.